One of the risks investing in real estate does carry is that it's hard to turn rentals into cash rapidly. Real estate is illiquid. However, i think this is where investing in real estate gets to be really creative. For example, if at the end of five years you need cash, you has numerous options since she has built up a good net worth in that time. You could:

sell two duplexes and pay off one of the remaining two entirely, so you would own one free and clear. Then you would have mortgage payments to make on just one. You would then clear $9,000 a years as cash flow plus the $1,350 she gets from the duplex with mortgage. You would also have realized at least $80,000 cash on the sale of two of your rentals. Of course you would own taxes on both the rental income and the capital gain from the sale. Still $80,000 cash isn't bad. You would have made back all of your original investment and more and would still have two duplexes

Refinance one, two, or more duplexes and use the tax free cash for anything you choose. Refinancing would increase your monthly debt service and increase the balance you owns on the loan. However you might use the money to purchase another property for cash with the though that the cash flow from a free and clear property would cover your increased debt service. The money from a refinance is tax free, a big point to remember.

Sell one, two or more duplexes and buy a big - 12 units or larger apartment buildings. Cash flow increase directly with the number of units per building, and you could sell one property, buy another using 1031 exchange to defer the capital gains tax.

However you need to always consult with your financial and tax advisor first before sell or refinance your rental properties.